8.9: Coastal Currents

Coastal currents are independent of and more variable than the adjacent oceanic gyre boundary currents. The zone in which coastal currents flow varies in width along different coasts. For example, the coastal current zone is wide off the Mid-Atlantic coast of North America because the continental shelf is wide, and the western boundary current (Gulf Stream) flows in deeper water off the edge of the shelf. In contrast, the coastal current zone off the Pacific coast of North America is narrow because the continental shelf is narrow, and the relatively shallow eastern boundary current flows over the narrow shelf almost to the coastline. Variable winds, tides, freshwater inflow from rivers, friction with the seafloor, and steering by coastline irregularities, such as capes, affect currents more in coastal waters than in the open ocean.

Coastal currents generally flow parallel to the coastline in a direction determined by the winds (and by Ekman transport). Coastal winds are more variable in speed and direction than are trade winds and other global winds that drive the ocean gyres. Coastal currents can be established quickly in response to storm winds, but they can also disappear or change direction within a few hours.

The strongest coastal currents occur when strong winds blow in areas with large freshwater inputs from rivers. In such areas, a shallow, low-salinity layer of surface water is formed over a very steep pycnocline. The surface layer slides easily over layers below this pycnocline. Thus, the wind energy is concentrated in the shallow, surface-layer current, rather than being transmitted and distributed throughout a greater depth.

Because they are directed by local wind patterns and interaction with the coastline, coastal currents may flow in the opposite direction from the adjacent ocean gyre boundary currents. For example, coastal currents off the Atlantic coast of North America generally flow to the southwest from Labrador to Cape Hatteras and along the Carolina, Georgia, and Florida coasts (Fig. 8-14) in the direction opposite that of the adjacent Gulf Stream.

Figure 8-14. Off the Atlantic coast of North America, the coastal currents are cold-water currents that flow to the south on the continental shelf inshore from the northward-flowing Gulf Stream.

The boundary between the warm Gulf Stream and colder coastal water, called a front, can be clearly discerned in satellite imagery as a distinct difference in water color. The nearshore water is greenish or brownish, whereas Gulf Stream water is a clear, deep blue. The color difference is due to the greatly reduced concentrations of suspended sediment and phytoplankton in Gulf Stream water (Chap. 5).

Off the west coast of North America, the boundary between the water of the California Current (which is part of the North Pacific Ocean subtropical gyre) and the coastal water is not well defined. The main reason is that the California Current is a weak, diffuse eastern boundary current. During winter and spring, a weak and variable coastal current, the Davidson Current, flows north in the opposite direction from the adjacent boundary current. The Davidson Current is driven by the predominant winds of winter and spring that blow from the southwest. These winds produce Ekman transport of water onshore and a geostrophic flow to the north on the resulting sea surface slope (Fig. 8-15a). In summer and fall, prevailing winds change to blow from the north. They cause Ekman transport offshore, a generally southward-flowing coastal current, and upwelling (Figs. 8-13, 8-15b). The summer and fall upwelling is responsible for the very high primary productivity and abundant sea life in coastal waters off the west coast of North America.

Figure 8-15. Coastal currents reverse seasonally off the Pacific coast of North America. (a) They flow to the north during winter and sometimes spring, when there is an area of atmospheric low pressure offshore and the coastal winds blow mainly from the southwest. (b) In the summer, when there is atmospheric high pressure offshore and coastal winds are mainly from the north, the coastal currents flow to the south, in the same direction as the offshore southward current that is part of the subtropical gyre in the North Pacific Ocean.